The present invention relates, in general, to radiation detection and, in particular, to a new and useful system for detecting and imaging low levels of gamma radiation.
In the radiation detection field, gamma imaging cameras have been utilized using both single-hole and multiple-hole collimators having a position-sensitive single detector. Several known gamma radiation detectors are disclosed in Redus et al., "A Nuclear Survey Instrument With Imaging Capability", IEEE Transactions on Nuclear Science, Vol. 39, No. 4, 1992; and Bird et al., "Images Obtained With A Compact Gamma Camera", Nuclear Instruments and Methods in Physics Research A299 (1990), p. 480-83.
It has been established that sensitivity to low gamma flux and spatial resolution of high radiation areas within the field of view is limited with these existing designs. Thus, it is known in this field that improved designs are needed which can detect low levels of gamma radiation and pinpoint the source of the radiation within the camera field of view.
Since refracting or reflecting materials are not available to focus short-wavelength gamma radiation, imaging must be performed by limiting the field of view of the detector. This is achieved by employing a lead piece having a hole therethrough known as a collimator. The detector is located at one end of the hole and observes only those gamma rays which pass through the hole. Gamma rays passing into the lead are absorbed.
One problem encountered when using a lead piece collimator having a single hole is that only a small area in the object field can be imaged at any given time. In order to speed up the inspection time for large areas, it is desirable to maximize the viewed area. One way this is achieved is to put multiple holes through the lead. However, until recently, large area detectors with spatial resolution have not been available. A single-large-area detector looking at all the holes in the lead is not able to discriminate or pinpoint which hole(s) the gamma rays had passed through.
One recent development in this field is to use position-sensitive photomultiplier tubes and charge-coupled device (CCD) arrays for gamma detection by placing a scintillator plate between the collimator and the detector. This method has proved effective for imaging regions in the object area with high gamma flux where the scintillator plate is thin. However, in order to detect low gamma flux, thicker scintillators are needed which increase the stand-off distance between the collimator and the detector. The effect of increasing stand-off distance is to increase the effective area (blur circle) seen by each spatial region of the detector, which degrades the spatial resolution of the system in the object area.